Best way to find long lost family?

So my dad was in one of those mother and baby homes in Ireland and never knew his family. I got him an ancestry.ie kit for his bday and he found his brother through loads of family trees. Only thing now is that the brother is being a bit annoying and won’t speak to him. He wants to find his sisters but obviously because they may be married and now have new last names we are finding it impossible... where do you go from here? Is it an invasion of privacy to look for his family tree?
Answers

susan

The good news is, his sisters are not likely to have the same reaction as the brother. If your dad can find just one sister, she will probably connect him to the others. Maybe you could hire a private detective. Give him the info on the brother that has already been found, and the heads up that that brother doesn't want to be contacted, and let the PI find out where that brother's sisters are.

Grace

Of course it isnt an invasion of privacy, its his family. Maybe one of his sisters still isn't married and can tell him where his other sisters are.